1. Field of the Invention
The present invention relates to a paper sheet pickup device for picking up a plurality of accumulated paper sheets one by one.
2. Description of the Related Art
A paper sheet pickup device is known, in which a belt with holes is run along paper sheets to hold them on the belt using a suction force applied thereto through a suction nozzle provided at the reverse side of the belt, thereby picking up them one by one (see, for example, U.S. Pat. No. 5,391,051). In this device, a solenoid valve is provided between the suction nozzle and a vacuum tank.
In this structure, when picking up paper sheets, the belt is run, the solenoid valve is opened, and the suction nozzle is operated to hold each paper sheet on the belt using a suction force. Further, when continuously feeding paper sheets, the solenoid valve is closed regularly in accordance with the feeding timing of each paper sheet, thereby providing gaps between sequentially fed paper sheets.
However, in the above structure, even if the solenoid valve is closed to stop suction by the suction nozzle, the negative pressure exerted on a paper sheet cannot quickly be eliminated where the paper sheet is held by the belt. Accordingly, even if the on-off cycle of the solenoid valve is shortened to feed paper sheets at high speed, high-speed feeding of paper sheets cannot be realized since the negative pressure exerted on the paper sheets cannot quickly be eliminated. This being so, paper sheets cannot quickly be picked up with certain gaps provided between them. Further, when the negative pressure cannot instantly be eliminated, simultaneous pickup of two stacked paper sheets will easily occur.
FIGS. 18 and 19 are schematic views of a conventional solenoid valve 100. FIG. 18 shows a state in which the solenoid valve 100 is open, and FIG. 19 shows a state in which the solenoid valve 100 is closed.
In general, the solenoid valve 100 comprises a coil 104 for axially moving a substantially cylindrical plunger 102, a chamber 106 (only shown in FIG. 18) containing the plunger 102, and two holes 108a and 109a formed in the bottom of the chamber 106 through which two tubes 108 and 109 are connected to the chamber. If the solenoid valve 100 is used in the above-mentioned apparatus of U.S. Pat. No. 5,391,051, the suction nozzle and the vacuum tank are connected to the two tubes 108 and 109, respectively.
When opening the solenoid valve 100, current is supplied to the coil 104 to pull the plunger 102 out of the chamber 106 and cause the two holes 108a and 109a to communicate with each other via the chamber 106. In contrast, when closing the solenoid valve 100, the supply of current to the coil 104 is stopped to push the plunger 102 into the chamber 106 and bring the bottom of the plunger 102 into contact with the bottom of the chamber 106. As a result, the two holes 108a and 109a are blocked, and a fluid channel 110 connecting the two tubes 108 and 109 is blocked.
The solenoid valve 100, however, has great inertia since it is opened and closed by axially moving the plunger 102. If the diameter of the tubes 108 and 109 is increased to increase the amount of introduced air, it is necessary to increase the diameter of the plunger 102 for blocking the holes 108a and 109a, and hence the solenoid valve 100 will have still greater inertia.
Further, when the solenoid valve 100 is opened, much time is required until air flows into the chamber 106 to make the pressure therein reach a preset value, after the coil 104 is energized to move the plunger 102. Namely, the response of the solenoid valve 100 is slow until air starts to circulate after power is supplied. In contrast, when the solenoid valve 100 is closed, the plunger 102 moves slowly since it is pushed into the chamber 106 against the preset pressure therein. Namely, the conventional solenoid valve 100 slowly operates when the coil 104 is energized and de-energized.
Therefore, if the solenoid valve 100 is used between the suction nozzle and the vacuum tank as in the mail feeding apparatus disclosed in U.S. Pat. No. 5,391,051, high-speed pickup of paper sheets cannot be realized because of the previously mentioned problem concerning elimination of negative pressure, and also because of the slow response of the solenoid valve 100 itself.
In addition, if the solenoid valve 100 is used in the mail feeding apparatus of U.S. Pat. No. 5,391,051, it is difficult to hold a relatively large and heavy paper sheet on the belt with holes, using vacuum pressure introduced through the holes. To be more specific, when the solenoid valve 100 is open, it is necessary to circulate air through a channel bent at several positions as shown in FIG. 18, which causes high passing resistance and hence makes it difficult to increase the flow of the air. This means that it is difficult to draw a relatively large amount of air through the suction nozzle, and therefore to hold a heavy paper sheet using vacuum pressure.